Observations of the cosmic microwave background represent a remarkable source
of information for modern cosmology. Besides providing impressive support for
the Big Bang model itself, they quantify the overall framework, or background,
for the formation of large scale structure. Most exciting, however, is the
potential access these observations give to the first moments of cosmic history
and to the physics reigning at such exceptionally high energies, which will
remain beyond the reach of the laboratory in any foreseeable future. Upcoming
experiments, such as the Planck mission, thus offer a window onto the Physics
of the Third Millennium